During operation of an electric generator, the relative rotation of the magnet poles of a field arrangement and the windings of an armature arrangement results in electric currents being induced in the windings. A large generator such as a wind turbine generator can comprise several hundred magnet poles with strong magnetic fields, and the currents induced in the windings are correspondingly large, so that the windings become very hot. The high temperatures can have a detrimental effect on the magnets as well as on other components in the generator such as wiring, control circuitry, sensors, etc. For this reason, a wind turbine generator is usually equipped with a cooling arrangement to cool the hotter parts of the generator. In some designs, heat exchangers are used to transfer the heat to a cooling fluid that circulates through pipes or hoses arranged throughout the generator. However, it is complicated and expensive to arrange such a fluid cooling system so that it efficiently draws the heat away from the windings. Another type of cooling arrangement may comprise a heat exchanger mounted at the rear of the wind turbine nacelle so that it can be cooled by the air passing over the wind turbine. Heat can be transferred from the hot components using cooling fluid in pipes or ducts to the exterior heat exchanger. The extensive arrangement of tubes, hoses, heat exchangers etc. adds to the overall complexity and cost of the wind turbine, and great care must be taken to avoid leakages. Furthermore, maintenance of such cooling systems adds significantly to the costs. In another approach, air may be drawn into the nacelle or canopy using a fan to generate an overpressure in the canopy, so that the air is compelled to pass over the generator in order to reach the exterior again, for example by escaping through a gap between the hub and the canopy. A disadvantage of such systems is that the air will always follow the ‘easiest’ or widest path to the exterior when it has to make its own way out. Therefore, such known systems are limited in their ability to lower the temperature at the source of the heat, i.e. the windings, since the spaces about the windings are narrow, as is the air-gap between windings and magnet poles. Air that is on its way to the exterior will tend to bypass such bottlenecks. Therefore, such air-cooling systems are generally inefficient. The inability to effectively cool the hot windings means that a wind turbine may not always be operated at full power, since the resulting high temperatures would damage the generator or generator components.
A wind turbine must be designed to operate reliably in different types of environment and under different weather conditions. A high relative humidity of the air in the wind turbine may cause problems, particularly if water vapour should condense on relatively cool components inside the generator. For example, in a direct-drive wind turbine, the outer rotor with its magnetic poles may initially be the coolest part of the generator, and condensation may form on the magnets as the temperature inside the generator increases. To address this problem, in one approach a dehumidifier may be used to extract water vapour from air that is fed into a closed chamber enclosing the stator. To be effective, an air seal between rotor and stator is also required so that the dry air also passes over the magnets. Such a system is considerably more complex and expensive to realise and to maintain, since it is not easy to seal off the rotor and stator together.